Warning: Some spoilers ahead. Though if you've either read the book or seen the movie, you're cool.
Order matters. It's not true what they say: that as between a book and a movie, you inevitably prefer whichever one you read or watch first. But the order you take them in nonetheless profoundly affects the experience. You can try to be objective – claim that each work has to stand or fall on its own merits, other incarnations be damned – but it won't work. You've been tainted.
I liked The Ruins – the movie. It was tight, brutal, ruthlessly effective; along with The Strangers, one of the year's few R-rated breaths of fresh air. Though it hewed pretty closely to genre conventions, it also recombined them to come up with its own interesting take on survival horror. I appreciated the movie's simplicity (the vines are a pure, almost elemental villain); its gruesomeness that never turned into sadism or needless cruelty; its grim, harsh relentlessness. It was a gripping roller coaster of a movie; a fun ride I enjoyed, praised, and pretty much put out of my mind.
Now that I've read the book, I ask myself: Would I still have liked the movie had I gone to the book first? The answer, I think, is no. It's not that I now think I was wrong about the film; to the contrary. But Scott Smith's novel is so extraordinary a genre achievement that the movie – adapted by Smith himself – can, in retrospect, feel only like a hapless abridgement, a wispy simulacrum of the novel's all-encompassing sense of doom and spiraling psychological terror. Taking the two in reverse order would have made the film feel cheap, impotent, lame; The Ruins for Dummies.